As is known, conventional bicycles are provided with a plate which is duly stiffened to the pedal shaft, from which the movement supplied to the pedals is transmitted, through a chain, to a pinion related to the rear wheel of the bicycle, which pinion is aided by a pawl system which allows the free rotation thereof in a specific direction.
On the other hand, and in order to vary the relationship of the transmission, in some cases the pinion related to the wheel is multiple, i.e., it is comprised of several pinions with a different diameter and consequently with a different number of cogs, and in other cases the plate related to the pedal shaft is likewise double or multiple, such that depending on the plate-pinion relationship in which the chain is established, a specific transmission relationship will in turn be established.
Given that the separation between the wheel shaft and the pedal shaft is constant, which is also the case of length of the drive chain, whereas the use of different rim-pinion groups will theoretically need chains with a different length, and furthermore, in any of the cases, the chain must be overdimensioned in order to allow the changes of pinion or rim, it is necessary to use tensile means to prevent uncoupling thereof, i.e., uncoupling thereof with respect to the elements which it couples to each other.
All of this brings about the use of complex change mechanisms, which require a high degree of precision, and which nevertheless do not eliminate the notable risk, both in changes and in the normal circulation of the bicycle, in the event of a hole in the road or due to any other circumstance, of the chain being uncoupled.